New Desalination Technique Also Cleans and Disinfects Water

Electrodialysis has the potential to desalinate seawater quickly and cheaply but does not remove other contaminants such as dirt and bacteria. Now chemical engineers have worked out how to do that too.

February 11, 2014

One of the world’s most pressing needs is to supply clean drinking water to the its population. In rural areas, almost half the population does not have access to clean water so the challenge is clear and present.

The problem, of course, is that most of the planet’s water is saline. So finding ways to desalinate seawater is a key goal.

Martin Bazant, postdoc Daosheng Deng, and colleagues at the Massachusetts Institute of Technology in Cambridge, say they’ve developed a new way to desalinate water, known as shock electrodialysis, that not only removes salt but particulate matter and bacteria too. “Shock electrodialysis has the potential to enable more compact and efﬁcient water puriﬁcation systems,” they say.

One of the big problems with desalination is its cost. The most common method is to distill seawater in a vacuum so that its boiling point is lower than usual. However, this is an energy intensive process that is expensive. So engineers are constantly on the lookout for cheaper methods.

The most common of these is reverse osmosis. This works by pumping water through a membrane that does not allow sodium or chlorine ions to pass. That’s significantly less energy intensive than traditional desalination methods but is limited by the rate at which water can pass through the membrane.

So in recent years, engineers have begun to study a process called electrodialysis. This works in the opposite way by allowing sodium and chlorine ions to pass through a membrane in the presence of an electric field, leaving purified water on the other side.

Because only the ions, rather than the water molecules, pass through the membrane, the rate at which this can desalinate is much higher than reverse osmosis.

But there is a problem with electrodialysis. Although it removes the salt from water, it does not remove other contaminants such as dirt and bacteria. So it requires additional stages of filtration and disinfection to make the water drinkable.

Now Bazant, Deng and co say they have found a way to produce clean drinking water in a single step using electrodialysis. The key is to place a layer of porous material close to the cathode which then acts as a filter and removes anything that cannot pass through the micropores.

The porous material in question is fitted glass, which is made by sintering together glass particles to form a porous solid. The pore size is around 0.5 micrometres so anything larger than that, such as dirt particles, cannot pass.

Bacteria tend to be smaller though. But Bazant and co show that these do not pass through the material either, probably because they get trapped or because they are destroyed by the powerful electric fields close to the cathode. “We were able to kill or remove approximately 99% of viable E Coli bacteria present in the feedwater upon flowing through the shock electrodialysis device with applied voltage,” they say.

Either way, the output from the new device is purified water with little if any of the original contaminants.

That’s a useful machine. Anything that can desalinate water while also filtering and disinfecting it is worthy of further study. Bazant and co suggest that it could be useful for chemical engineering.

The bigger question, of course, is whether such a device could cost-effectively purify water on a larger scale for those who need it most. That would mean using photovoltaic power, for example, and producing reliable flow rates over an extended period of time.

That’s another challenge entirely. Turning a lab-based prototype into a practical, reliable machine is just as hard again. But given the stakes, the lives of millions of people, it’s certainly worth aiming for.